This disclosure generally relates to an impact energy absorber and more particularly, to an impact energy absorber employing a magnetorheological fluid for selectively controlling a stroking force of the absorber.
Current impact energy absorbers generally have a fixed response and are not tunable. For example, some impact energy absorbers rely base their energy absorption on the crash (plastic deformation) of a honeycomb-like structure, whose response is fixed and non-tunable for each form of such material. However, these types of impact energy absorbers provide a one time response, are expensive to replace, and are not tunable. Those impact energy absorber systems that are tunable generally rely on hydraulics, which are relatively bulky and expensive.
For an example of a prior art impact energy absorber, U.S. Patent Publication No. 20030113160A2 filed on Dec. 19, 2001 describes a highway crash attenuator frame that includes one or more tension elements secured between opposed side elements near the respective central hinges. Each tension element extends across the longitudinal axis of the frame, from one side of the frame to the other side of the frame. Each tension element includes a mechanical fuse that fails in tension when the first and second side elements of the frame apply an excessive load to the tension element. Once the mechanical fuse fails, central hinges on both sides of the frame are simultaneously allowed to begin opening. In this way, the collapse of the frame is coordinated between the left and right sides of the frame. However, the use of a fuse is a one-time event, requiring expensive replacement.
Magnetorheological (MR) fluids belong to a class of controllable fluids. The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds when exposed to a magnetic field. In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian liquids.
A typical MR fluid has about 20 to about 40 percent by volume of relatively pure, soft iron particles, and a diameter of about 3 to about 5 microns suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles.
MR fluids made from iron particles typically exhibit maximum yield strengths of 30 to 90 kPa for applied magnetic fields of 150 to 250 kA/m (1 Oe . 80 A/m). MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and use. Furthermore, because the magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives, it is a relatively straightforward matter to stabilize MR fluids against particle-liquid separation in spite of the large density mismatch.
It is very desirable to have the ability to provide different responses based upon sensor input, such as, for example, a variable response based on vehicle speed and nature of the impacting object, so as to meet the differing energy absorption and stroking force requirements of different impact scenarios. Accordingly, there remains a need for a tunable energy impact absorbers that are inexpensive, easy to repair or replace, and can be used multiple times.